Vacuum cleaners are well known for collecting dust and dirt, although wet-and-dry variants which can also collect liquids are known as well. Typically, vacuum cleaners are intended for use in a domestic environment, although they also find uses in other environments, such as worksites or in the garden. Generally, they are electrically powered and therefore comprise an electric motor and a fan connected to an output shaft of the motor, an inlet for dirty air, an outlet for clean air and a collection chamber for dust, dirt and possibly also liquids. Electrical power for the motor may be provided by a source of mains electricity, in which case the vacuum cleaner will further comprise an electrical power cable, by a removable and replaceable battery pack, or by one or more in-built rechargeable cells, in which case the vacuum cleaner will further comprise some means, such as a jack plug or electrical contacts, for connecting the vacuum cleaner to a recharging unit. When the vacuum cleaner is provided with electrical power from one of these sources, the electric motor drives the fan to draw dirty air along an air flow pathway in through the dirty air inlet, via the collection chamber to the clean air outlet. The fan is often a centrifugal fan, although it can be an impeller or a propeller.
Interposed at some point along the air flow pathway, there is also provided some means for separating out dust and dirt (and possibly also liquids) entrained with the dirty air and depositing these in the collection chamber. This dirt separation means may comprise a bag filter, one or more filters and/or a cyclonic separation apparatus.
In the event that the dirt separation means comprises a bag filter, dirty air, which has entered the vacuum cleaner via the dirty air inlet, passes through the bag filter. This filters out, and collects within the bag filter, dust and dirt entrained with the dirty air. The filtered material remains in the bag filter which lines the collection chamber. The clean air then passes to the other side of bag filter and through a grille in the collection chamber under the influence of the fan. The fan draws air in and expels it out, from where the air then passes to the clean air outlet of the vacuum cleaner.
There is always a small risk of dust and dirt passing through the bag filter and it is undesirable that it be allowed to pass through the fan and cause damage. To reduce this potential problem, there is often a fine filter located across the grille of the collection chamber to remove any fine dust and dirt particles remaining in the air flow after passage through the bag filter. This is commonly known as a pre-fan filter.
Occasionally, and in addition to any pre-fan filter, there is a high efficiency filter located downstream of the fan before the air flow leaves the vacuum cleaner. This is to remove any remaining extremely fine particulate matter which will not harm the fan or motor, but which may be harmful to the household environment. The term “filtering efficiency” is intended to relate to the relative size of particulate matter removed by a filter. For example, a high efficiency filter is able to remove smaller particulate matter from air flow than a low efficiency filter. A HEPA filter is a high efficiency filter which should be able to remove extremely fine particulate matter having a diameter of 0.3 micrometers (μm) and lower.
The purpose of the bag filter is to filter dust and dirt entrained in dirty air flow and to collect the filtered material within the bag filter. This progressively clogs the bag filter. The volumetric flow rate of air through the vacuum cleaner is progressively reduced and its ability to pick up dust and dirt diminishes correspondingly. Hence, the bag filter needs replacement before it becomes too full and before vacuum cleaner performance becomes unacceptable. The volume of the collection chamber must be sufficiently large to merit the cost of regular bag filter replacement.
An upright vacuum cleaner commonly has an upright main body with a dirt separating means, a motor and fan unit, a handle at the top and a pair of support wheels at the bottom. A cleaner head with a dirty air inlet facing the floor is pivotally mounted to the main body. A cylinder vacuum cleaner commonly has a cylindrical main body with a separating dirt means, a motor and fan unit and maneuverable support wheels underneath. A flexible hose with a cleaner head communicates with the main body. Bag filters are commonly used in upright and cylinder vacuum cleaners as separation means because their main body has sufficient internal space for the large collection chamber required to accommodate the bag filter.
In the event that the dirt separation means comprises a filter, dirty air, which has entered the vacuum cleaner via the dirty air inlet, passes through the filter. This filters out dust and dirt entrained with the dirty air and the filtered material remains in the collection chamber on the upstream side of the filter. Sometimes the filter is supplemented by a sponge to absorb any liquids entrained in the dirty air flow. The clean air then passes to the other side of filter under the influence of the fan, and from the fan the air then passes to the clean air outlet of the vacuum cleaner.
Filtered material accumulates around, and progressively clogs, the filter. The volumetric flow rate of air through the vacuum cleaner is progressively reduced and its ability to pick up dust and dirt diminishes correspondingly. Hence, the collection chamber needs regular emptying and the filter needs frequent cleaning to mitigate against this effect. Sometimes, the vacuum cleaner has a filter cleaning mechanism. Alternatively, the filter needs to be removable for cleaning with a brush, or in a dish washer, for example.
Hand-holdable vacuum cleaners, as their name would suggest, are compact and lightweight and are intended to perform light, or quick, cleaning duties around a household. Typically, hand-holdable vacuum cleaners are battery-powered to be easily portable.
An example of a hand-holdable vacuum cleaner having the conventional motor, fan and filter arrangement is described in European patent publication no. EP 1 752 076 A, also in the name of the present applicant. This vacuum cleaner has dirty air inlet at one end of a dirty air duct leading to a collection chamber with a filter. The collection chamber is generally cylindrical and is arranged transverse the body of the vacuum cleaner. The dirty air duct is rotatable, with the collection chamber, in relation to the body. The dirty air duct may be adjusted to access awkward spaces while the vacuum cleaner is held comfortably by a user.
In the event that the dirt separation means comprises cyclonic separation apparatus, dirty air, which has entered the vacuum cleaner via the dirty air inlet, passes through the cyclonic separation apparatus having one or more cyclones. A cyclone is a hollow cylindrical chamber, conical chamber, frustro-conical chamber or combination of two or more such types of chamber. The cyclone may have a vortex finder part way, or all way, along its internal length. The vortex finder is commonly a hollow cylinder and it has a smaller external diameter than the internal diameter of the cyclone.
Dirty air enters via a tangentially arranged air inlet port and swirls around the cyclone in an outer vortex. Centrifugal forces move the dust and dirt outwards to strike the side of the cyclone unit and separate it from the air flow. The dust and dirt is deposited at the bottom of the cyclone and into a collection chamber below. An inner vortex of cleaned air then rises back up the cyclone. The role of a vortex finder is to gather and direct the cleaned air through an air outlet port at the top of the cyclone. As an alternative to a vortex finder, the cyclone may have an inner cylindrical air permeable wall providing the cleaned air with a path from the cyclone. From the cyclone the cleaned air passes, under the influence of the fan, to the clean air outlet of the vacuum cleaner.
As with a bag filter, a vacuum cleaner with a cyclonic separation apparatus may have a pre-fan filter to protect the fan and motor, especially if the air flow is used to cool the motor. Nevertheless, volumetric flow rate of air through the vacuum cleaner remains virtually constant as separated material accumulates in the collection chamber. Thus, an attraction of cyclonic separation apparatus in a vacuum cleaner is a consistent ability to pick up dust and dirt. Another attraction is that the cost of regular bag filter replacement is avoided.
An example of an upright vacuum cleaner having a motor, fan and cyclonic separation apparatus is described in European patent publication no. EP 0 042 723 A. This cyclonic separation apparatus is divided into a first cyclonic separating unit with a cyclone formed by an annular chamber and a second cyclonic separating unit with a generally frustro-conical cyclone. The first cyclonic separating unit is ducted in series with the second cyclonic separating, unit. Air flows sequentially through the first, and then the second, cyclonic separating units. The frustro-conical cyclone has a smaller diameter than the annular chamber within which the frustro-conical cyclone is partially nested. Separated material from both cyclonic separating units collects in the cylindrical collection chamber formed at the bottom of the annular chamber.
The term “separation efficiency” is used in the same way as filtering efficiency and it relates to the relative ability of a cyclonic separation apparatus to remove small particulate matter. For example, a high efficiency cyclonic unit can remove smaller particulate matter from air flow than a low efficiency cyclonic separating unit. Factors that influence separation efficiency can include the size and inclination of the dirty air inlet of a cyclone, size of the clean air outlet of a cyclone, the angle of taper of any frustro-conical portion of a cyclone, and the diameter and the length of a cyclone. Small diameter cyclones commonly have a higher separation efficiency than large diameter cyclones, although other factors listed above can have an equally important influence.
The first cyclonic separating unit of EP 0 042 723 A has a lower separating efficiency than the second cyclonic separating unit. The first cyclonic separating unit separates larger dust and dirt from the air flow. This leaves the second cyclonic separating unit to function in its optimum conditions with comparatively clean air flow and separate out smaller dust and dirt.
A hand-holdable vacuum cleaner having a motor, fan and cyclonic separation apparatus is described in United Kingdom patent publication no. GB 2 440 110 A. This cyclonic separation apparatus is smaller than that of EP 0 042 723 A in order to be used in a hand-holdable vacuum. It is divided into a first cyclonic separating unit and a second cyclonic separating unit located downstream of the first cyclonic separating unit. The separating efficiency of the first cyclonic separating unit is lower than that of the second cyclonic separating unit.